This invention generally relates to a high efficiency power amplifier system. More specifically, this invention relates to a power amplifier with a variable impedance network coupled to an output of the power amplifier.
The power amplifier is a key technology in portable radiotelephone design. In cellular telephones, the power amplifier has a large impact on the available talk time. This is because the power amplifier consumes a significant amount of power relative to the other circuitry within the cellular telephone. One parameter defining how much power the power amplifier consumes is the power amplifier efficiency. This efficiency, for example can be the power added efficiency. For power amplifiers using bipolar devices, a collector efficiency is another efficiency parameter. For FET-based power amplifiers, the drain efficiency is an additional parameter used to characterize amplifier efficiency.
Attempts have been made to increase the efficiency of power amplifiers by varying the load impedance presented to the power amplifier. U.S. Pat. No. 5,673,001 (hereinafter ""001) is shows several examples of load switching. In a first embodiment, the radio frequency (RF) input signal and the RF output signal are sampled. A control circuit develops a control signal based on the input-output signal comparison. A voltage variable capacitor (VVC) at the output of the power amplifier is varied in response to the control signal to skew the operating point of the power amplifier closer to a maximum efficiency operating point.
In a second embodiment of the prior art, only a sampled portion of the RF output signal of the power amplifier is used to generate the load control signal for control of the VVC at the power amplifier output. Once again, a sample of the power of the output signal is used to control the impedance of presented by the VVC.
In a third embodiment of the prior art, a saturation detection loop is utilized. In cellular systems, there is typically an output power control loop in the transmitter, and the output power is varied over a range of output powers. For example, if the mobile unit is close to the base station, the mobile unit will not have to transmit as much output power. In the third embodiment of the prior art, the output signal power is measured to develop the control signal. The voltage of the control signal is compared to the supply voltage, and if the control signal voltage is within a predefined range from the supply voltage, the VVC impedance is adjusted. Without this saturation detect loop, the VVC network could adjust to a high efficiency impedance but set the impedance to one for which the PA cannot deliver the necessary output power. Thus, the VVC could present an impedance to the power amplifier output to cause the power amplifier to saturate at an output power that is lower than the desired output power.
The embodiments described in the prior art are very useful for constant envelope modulation schemes such as frequency modulation in the advanced mobile phone system (AMPS) or gausian minimum shift keying (GMSK) in the group special mobile (GSM) digital cellular telephone system. However, other digital cellular telephone systems employ modulation schemes that have an amplitude modulation (AM) envelope impressed upon the RF modulated signal as opposed to a constant envelope.
For example, the cellular telephone interim standard (IS)-136 utilizes time division multiple access (TDMA) and requires xcfx80/4 differential quadrature phase shift keying (DQPSK), and the cellular telephone standard (IS-95) utilizes code division multiple access (CDMA) and requires offset QPSK (OQPSK). Yet another cellular telephone system employs quadrature amplitude modulation (QAM) that also has an AM envelope on the RF modulated signal.
The prior art systems rely upon real-time measurements to vary the load impedance as a function of output power. That becomes inadequate for linear modulation schemes. The prior art systems do nothing to address the adjacent channel power (ACP) that is transmitted by the power amplifier. In systems using linear modulation, if the power amplifier does not faithfully reproduce the AM envelope, the power amplifier will transmit too much ACP. The radiotelephone employing the power amplifier would then not comply with the cellular standard requirement for ACP. Thus, the prior art adjusts the load impedance based on real time measurements. This is a simple load adjustment which does not address adjacent channel power performance.
In addition, the closed loop impedance control systems of the prior art do not provide for the best impedances for all variations of operating frequency and output power. This is because the algorithm to control the VVC circuit is rather simple in that a simple up direction or down direction is fed to the VVC circuit based on an output power detection. Part to part variations of the power amplifier circuitry used in each radiotelephone may also reduce the efficiency of the power amplifier.
Accordingly, there is a need for power amplifier load adjust system with more accurate and comprehensive control of the impedances presented to the power amplifier output by the variable impedance network. There is a further need for the load adjust system to compensate for part to part variations. There is a further need to control the variable impedance network in a manner that controls the adjacent channel power transmitted by the power amplifier.